That's very good, considering the fortwo is only rated at 41 highway.
Not by hypermilers. It's a hypermiler's dream with a transmission you can coast with whenever you want. I can break 60mpg with a tailwind.
And I never imagined towing with it. I wouldn't hesitate to do modest towing with anything I own, but the notion of doing it with a smart never even crossed my mind.
You aren't the only one.
Did it behave okay under load?
Wonderful.
I understand the slow-shifting transmission is less than desireable for spirited motoring of any kind; was it a complication for load-centric issues like climbing mountains?
No. It was a lot like the 2,000,000 miles I drove on heavy trucks. I'm very gentle with the throttle. I baby the car, all the time. The transmission is marvelous for heavy loads or hypermiling. For spirited driving I have a V12 BMW and a V8 Jaguar. But you can't have more fun on Hy 80 than with a Smart Car pulling a Wildfire on a trailer.
Everybody drives up along side you, waves and takes pictures.
Smart Car with Trailer: CA to IL | CleanMPG
BTW, a Smart is pretty frisky when you manual shift with high revs.
Too bad it requires premium. Your dollar savings could have been even higher.
I'm still not convinced of that. The strong Mitsubishi 3B21 long-stroke engine was designed for premium. Although it will run fine on regular unleaded, the power stroke is longer and more even using premium, which burns slower in the combustion chamber, resulting in more torque.
But hey, 50mpg is saving money no matter how you slice it. Nice results.
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What's it rated to tow?
It's not. Zero. You can't even tell a canoe or the alumionmum Featherlite trailer is back there. But with the heavy loads I haul, most people would probably break something when they got impatient. I don't get impatient (but the people behind me do!) 
Installing the great Curt hitch on a Smart Car:
https://youtu.be/G07EteeOfwA
BTW, I'm in California where you pay an arm and a leg to register a pickup truck. I can move anything I ever hauled in my pickup bed and I don't have to lift it up nearly as high to load it in. There's no trailer fees in California for a little trailer like this, so I save a hellova lot more money! Every time a pickup truck driver shakes his head or gives me the finger or his buddy hangs his ass out the passenger window (I've seen it all), I just cackle like a fool with a big secret.
Let's see you do this with your pickup! Loading an 800-pound boulder onto the trailer at the Smart Quarry. They told me to hitch a cat to it but I only had a dog...